diff --git a/src/Azure/PubSub/PubSubManager.php b/src/Azure/PubSub/PubSubManager.php
index 5642081..c5427c9 100644
--- a/src/Azure/PubSub/PubSubManager.php
+++ b/src/Azure/PubSub/PubSubManager.php
@@ -48,7 +48,7 @@ public function __construct(
   }
 
   /**
-   * Joins a group.
+   * Initializes the websocket client.
    *
    * @throws \JsonException
    * @throws \WebSocket\ConnectionException
@@ -60,6 +60,9 @@ private function initializeClient() : void {
     }
     $client = $exception = NULL;
 
+    if (empty($this->settings->accessKeys)) {
+      throw new ConnectionException('PubSub access key is undefined.');
+    }
     // Initialize client with primary key, fallback to secondary key.
     foreach ($this->settings->accessKeys as $key) {
       $exception = NULL;
diff --git a/tests/src/Unit/Azure/PubSub/PubSubManagerTest.php b/tests/src/Unit/Azure/PubSub/PubSubManagerTest.php
index 30f218e..9ae7b7d 100644
--- a/tests/src/Unit/Azure/PubSub/PubSubManagerTest.php
+++ b/tests/src/Unit/Azure/PubSub/PubSubManagerTest.php
@@ -89,7 +89,24 @@ public function testInitializeClientInvalidCredentials(): void {
   }
 
   /**
-   * Tests sendMessage() and clientText() methods.
+   * Tests initializeClient() with empty access keys.
+   */
+  public function testInitializeClientNoSettings() : void {
+    $this->expectException(ConnectionException::class);
+    $this->expectExceptionMessage('PubSub access key is undefined.');
+    $sut = new PubSubManager(
+      $this->prophesize(PubSubClientFactoryInterface::class)->reveal(),
+      $this->prophesize(EventDispatcherInterface::class)->reveal(),
+      $this->prophesize(TimeInterface::class)->reveal(),
+      new Settings('', '', '', []),
+      $this->prophesize(LoggerInterface::class)->reveal(),
+    );
+    // Make sure initialize client fails when settings are empty.
+    $sut->sendMessage(['test' => 'something']);
+  }
+
+  /**
+   * Tests sendMessage() method.
    */
   public function testSendMessage() : void {
     $time = $this->prophesize(TimeInterface::class);
@@ -121,7 +138,7 @@ public function testSendMessage() : void {
   }
 
   /**
-   * Tests receive() and clientReceive() methods.
+   * Tests receive() method.
    */
   public function testReceive() : void {
     $expectedMessage = '{"message":"test"}';